This disclosure teaches a new process and apparatus for producing graphic images on substrates, using photopolymers such as paste-consistency ultraviolet light (UV) curable photopolymers. These photopolymers are characterized by their composition of 100 percent reactive polymers, which are transformed from a paste-consistency wet film to a dry coating by exposure to a strong UV light source for several seconds. The photopolymers as used in the disclosed process are further characterized as being imaging quality, or capable of being selectively hardened by light passing through a photographic master, thereby producing a film securely affixed to a substrate, at locations where the photographic master allows the UV light to impinge on the photopolymer.
The photopolymers which can be imaged by the disclosed process and apparatus are of known composition, a representative list being included herein. These photopolymers are available commercially as plating resists and as etch resists for use in printed wiring board (PWB) manufacture, for example. These photopolymers are also used extensively in graphics imaging and are being applied conventionally by screen printing.
This class of photopolymers has been developed to be applied to substrates via screen printing, wherein the images are deposited on the substrates as wet photopolymer, and are then hardened and transformed into permanent images by being subjected to a strong UV light source.
The screen printed photopolymer images are characterized by indistinct boundaries, loss of fidelity, smeared images and limited film thickness, not so much because of the photopolymer viscosity characteristics as the inherent limitations of screen printing art as achieved in a production environment by semi-skilled printers.
The images which can be achieved using the disclosed process and apparatus and the same photopolymers are characterized by lines having sharp, distinct boundaries, exceptional fidelity with film thicknesses up to 0.002 inches (0.5 mm) and no smearing. For example, screen printed PWB resist patterns are practically limited to conductor widths and spacing of 0.010 inches (2.54 mm) minimum, while the same photopolymers can be imaged as disclosed herein to produce line widths and spacings of 0.003 inches (0.076 mm), with a film thickness of 0.00025 inches (0.006 mm).
Screen printed half-tone images are limited to a practical upper range of 105 lines, with dot sizes of 20 to 80 percent. The same photopolymer imaged as described herein can produce half-tone graphics of 150 lines, with dot sizes of 5 to 95 percent.
In addition to improving on the quality of images produced by screen printing, the disclosed process and apparatus produces superior images as compared with images produced by the dry film photoresists such as in widespread use in the manufacture of printed wiring boards.
Dry film photoresists are laminated onto the PWB by heat and roller pressure; exposed to a UV light source through a photo image, hardening the exposed photopolymer to the extent that the subsequent solvent washout step ideally removes only unexposed photopolymer. The images produced are of excellent definition, having distinct boundaries uniform thickness and a resolution down to 0.005 inches (0.13 mm) line widths. Dry film photoresist images are expensive to produce owing to the cost of photopolymer, the necessity to discard twice as much plastic carrier film as applied photopolymer; and the need for expensive laminating, exposing and washout equipment.
In addition to screen printed images and dry film photoresist images, a third imaging method is known but not in widespread use, wherein PWB solder mask coatings are photoimaged by the Bell Laboratories. The process consists of roller coating the PWB surface with solder mask photopolymer, laminating a thin plastic sheet thereover and exposing the photopolymer through a phototool via a collimated light source. Use of atmospheric pressure to force the phototool into intimate contact with the thin plastic sheet would force or extrude the photopolymer into the drilled holes, and thins out the coating on the printed wiring. The thin plastic sheet also separates the emulsion of the phototool from the surface of the photopolymer so that loss of fidelity is experienced because of light undercutting.
While this invention is advantageous in making printed wiring boards (PWB), it also in the same manner produces high resolution half-tone or line printing plates, with very simple equipment which is readily automated as compared with prior art systems.
Exemplary prior art for preparation of photopolymer printing plates includes U.S. Pat. No. 4,070,110--J. W. Olt, Jan. 24, 1978 for producing photopolymer relied printing plates in a semi-automated process. Thus a sandwich array is prepared by laminating a cover film and backing sheet with intermediate photopolymer applied in liquid form and hardened by partial exposure to light. The sandwich is after being completely formed, flattened under pressure, while exposing through the phototransparencies. A complex machine array for a similar laminating and followthrough developing process is shown also in U.S. Pat. No. 4,087,182--H. Aiba et al., May 2, 1978. These are contact printing processes as compared with a photo imaging process such as described in U.S. Pat. No. 4,052,603--K. Karlson, Oct. 4, 1977.
All of these involve complex and expensive machinery and do not result in simple single pass lamination and exposure scanning as does the present invention.
Other methods of sandwiching and developing photopolymer plates have been devised such as set forth in U.S. Pat. No. 3,848,998--S. Yonekura et al., Nov. 19, 1974 using rigid plates.
However rigid plates cannot be used to achieve desired high resolution because of the difficulty at low expense to achieve optically flat surfaces and the dependency upon a uniform thickness and very smooth surface on the photopolymer being exposed.
Therefore the present invention has as a general object, the simplicity and improvement of prior art equipment and processes for producing printed elements of high resolution with photopolymers.